What are the differences from the Adv. Enterprise firmware and the Adv. IP Services?
From the 2610XM perspective.
In short, Advanced Enterprise combines the features of all other images in to one. You obviously have to buy this from your Cisco sales rep in order to use it. Most of the Enterprise features are more or less useless for a normal Internet-world network. IPX, SNA etc. are all very rare (except in old legacy enterprise networks). Enterprise usually takes more router memory as well, no need.
For full detauls about the 12.4 licensing / release schema: http://www.cisco.com/en/US/prod/collateral/iosswrel/ps8802/ps5460/prod_bulletin0900aecd80281b17.html
The feature navigator has the full list of features in each release:
For the most part, HampusLi's answer is correct. Some special features such as lawful intercept may not be included in ADVANCED ENTERPRISE SERVICES package.
You can always use Cisco Feature Navigator http://tools.cisco.com/ITDIT/CFN/ to make sure. I don't know why Cisco removed package comparison from the download page.